Ink jet printing as it is known, provides a rapid and quiet method of printing with tiny drops of ink. The ink drops are ejected from orifices in an ink head which is closely spaced (commonly of the order of about 1 to about 2.5 mm for impulse ink jets, but longer distances for continuous ink jets) from the record medium on which characters or other information is to be printed. The characters are formed by small drops which, depending upon a variety of well known factors such as the ink and paper, result in varying degrees of printing quality.
An early article on ink jet printing is entitled "High Frequency Recording With Electrostatically Deflected Ink Jets" by R. G. Sweet and published in The Review of Scientific Instruments, Volume 36, No. 2, Pages 131-136, February 1965 and published by the American Institute of Physics. This article describes one type of ink jet printing, known as a continuous jet, in which a single or an array of orifices generate a continuous stream of drops at a relatively high speed (of the order of 20 m/sec) and a frequency of order of 100 KHz with different and higher frequencies being possible.
In order to print with the continuous ink jet, an electrostatic deflection technique is employed whereby the ink drops as they leave the nozzle or orifice have been given an electric charge. A deflection is then obtained by subjecting the charged drops to a uniform electric field.
The continuous ink jet enables very high speed printing, though often at the expense of quality in the printed character and requiring an elaborate control system. For example, when a high speed ink jet is used, care must be taken to avoid the electric charge of one drop affecting the trajectory of the adjacent drop. Furthermore, the continuous ink jet requires a high pressure ink supply, of the order of 50 lb/in.sup.2 which demands more expensive feed lines and pressure regulation to maintain a proper feed of ink. The ink for a continuous ink jet must have a certain conductivity to be charged at the orifice. This limits the types of inks which can be used and thus the printing quality when a paper to be printed requires use of an ink which is not sufficiently conductive. Notwithstanding these difficulties, continuous ink jets have been successfully employed in high speed printers.
Another technique for ink jet printing is known as an impulse ink jet in which ink is supplied at a very low pressure, usually of the order of several inches of water, to a capillary tube ending at an orifice. An impulse generator such as a piezo-electric device is used to cause a pressure pulse through the capillary tube to the orifice and thus eject a drop of ink. Usually a print head will contain an array of orifices, each being supplied with ink through a capillary tube and having its own impulse generator. Electrical control over the impulse generators enables the formation of characters on a record medium. A description of an impulse jet printing head may, for instance, be found in an article entitled "Silent Ink Jet Printing For Printer Terminals" by J. Heinzl et al. and published in Siemens Review 44(9) pages 402-404, September 1977.
The impulse jet printing technique commonly involves printing with ink drops ejected at speeds of the order of 1-3 m/sec (about ten times slower than a continuous ink jet) and with a substantially lower drop frequency than with a continuous ink jet.
Although an impulse ink jet prints at a slower speed, it has advantages of a simpler control, a lower operating pressure and has a potential for producing quality printed characters. An improved control and drop consistency is obtained by operating the impulse ink jet continuously. In such case, however, a deflection control is required for the electrically neutral drops.
The U.S. Pat. No. 3,871,004 to Rittberg describes a technique for deflecting an array of electrically neutral ink drops by producing a potential gradient adjacent the orifices of an ink jet print head. The ejected electrically neutral ink drops are influenced by the potential gradient and are deflected towards the region of higher electric field intensity. With a deflection control as described in the Rittberg patent, control over individual drops is not provided and the described structure does not lend itself readily to cleaning of the orifices and adjacent ink head surfaces.